The present invention relates to the formation of water-based, clay-free drilling fluids containing an effective amount of ferric hydroxide agent as the agent to impart viscosity and pseudoplastic properties to the fluid and to methods of using such fluids in the drilling of subterranean soil.
In normal well drilling operations in which a bore hole is drilled by a rotary method, the bore hole is generally filled with a drilling fluid or mud which is circulated therein. Drilling fluids are usually pumped down through the drill stem of the rotary rig, circulated around the drill bit and returned to the surface through the annular passage between the drill stem and well casing. These drilling fluids perform a number of functions including lubricating the drill stem and bit, cooling the bit, carrying the cuttings from the bit up the bore hold to the surface where the cuttings may be separated and disposed of, and providing a hydrostatic head against the walls of the well to contain downhole geopressure.
A primary requisite of a satisfactory drilling fluid is its ability to readily circulate and flow, that is, to have low viscosity, under the high sheer conditions which are present in the environs of the drill bit while, at the same time, also being capable of exhibiting comparatively high viscosities and carrying power under varying low shear conditions which are exerted in the annular passage between the drill stem and the well casing during the fluid's upward flow movement. In generic terms, the drilling fluid must exhibit non-Newtonian properties as a pseudoplastic fluid, especially under varying low shear rates such as are encountered in the annular passage.
The drilling fluid should also be capable of exhibiting the above described pseudoplastic properties under changing conditions encountered during the drilling operation due to the fact that the bore hole traverses various strata such as shales, clay, etc., and the cuttings of these materials become dispersed in the fluid medium. The drilling fluid components should, therefore, be substantially stable to the presence of various calcium compounds and to sodium chloride which may be present in the fluid from the soil strata with which it is in contact and/or due to the use of salt water used to form the drilling fluid.
It is also well known that as the bore hole increases in depth the temperatures encountered substantially increase above that found at the earth's surface. This is especially a problem found in drilling to depths greater than about 9,000 feet as is presently becoming a more common practice. In addition, heat is generated by frictional forces on the drill bit. It is, therefore, not uncommon for the drilling fluids to encounter temperatures of 200.degree. F. or greater. Components used in the forming drilling fluids should be stable with respect to elevated temperature conditions.
A wide variety of drilling fluids have been used, including aqueous based liquids, hydrocarbon based liquids, air and other gases, mist, foams and the like. Since great volumes of drilling fluids are required for the purposes of providing a cooling medium for the rotary bit and a means of carrying off the drilled particles, most of the conventional fluids used have been based on water. Water, being a Newtonian fluid, does not have the needed capability to efficiently carry the drilled particles from the bore hole to the surface. This becomes increasingly apparent as the bore hole increases in depth.
It is a widely held and accepted theory that the viscosities suitable for creating a particle carrying capacity in the fluid can be achieved with a drilling fluid having pseudoplastic properties, that is, that the viscosity must be sufficient to prevent the drilled particles from separating from the drilling fluid when the fluid is under the influence of low shear forces such as encountered in the annular passage. In order to obtain the requisite pseudoplastic properties, it has been thought desirable to use certain clay or colloidal clay bodies such as bentonite or attapulgite clays. As a result, the drilling fluids have been usually referred to as "muds." The use of clay-based drilling fluids has provided the means of initially meeting the two basic requirements of drilling fluids, i.e., cooling and particle removal. However, these clay-based drilling fluids are highly unstable when they come in contact with various salts commonly found in drilled earth formations.
Materials which have come into expanding use as the viscosifying agent of drilling compositions are Xanthan gums such as are described in U.S. Pats. Nos. 3,198,268; 3,208,526; 3,251,147; 3,243,000; 3,307,016 and 3,319,715. These materials have been found to cause acqueous drilling fluids to exhibit pseudoplastic properties under varying low shear forces. These materials, however, whether used alone or in combination with other additives, are irreversibly degraded by the elevated tempertures often encountered during conventional drilling operations. The high cost of the Xanthan gums and the high rate of degradation limit their usefulness to specialized operations.
Iron compounds have previously been used in drilling fluids as additives to enhance certain specific property of the fluid. For example, iron oxides of magnetite and hematite have been added to drilling fluids to increase the overall density of the fluid. U.S. Pat. No. 2,868,726 to Bruker et al. discloses that a water soluble iron-containing compound may be added to lime muds to increase their high temperature gelation properties. U.S. Pat. No. 2,393,166 to Hoeppel discloses the use of water soluble salts of iron as an agent capable of inhibiting the hydroxyl ion concentration in the drilling fluid. These water-soluble products are not capable of imparting the highly desired pseudoplastic properties.
There is a general need for a drilling fluid viscosifier which is capable of being produced at low cost, which renders an aqueous drilling fluid pseudoplastic so as to exhibit ease of fluidity under high shear condtions such as found at the drill bit site while exhibiting an increased, substantially stable viscosity under low shear conditions and which is substantially stable under conditions normally encountered in drilling a bore hole.